1. Field of Invention
The present invention relates generally to generating clock signals for electronic devices and, more particularly, to methods and apparatus for generating a clock signal from an AC power signal.
2. Discussion of Related Art
In order to operate effective and accurately, electronics devices often require or make use of internal “clocks” or crystals which oscillate at a certain fixed frequency. In addition, some electronic devices, such as computers, clock radios, and the like, keep time in the form of a digital clock. These devices rely on a variety of methods to “check the clock” in order to verify that the clock is not running too quickly or too slowly. The clock checking mechanism may be internal (e.g., using circuitry, such as a crystal oscillator, within the device) or external (e.g., the device receives a clock signal from an external source).
Many electronics devices get their power from a wall outlet. Power utility companies are very careful to ensure that the power at the wall outlet oscillates at a very precise frequency, as any deviation in the frequency can disrupt the power grid. Accordingly, the power line, providing typically either a 50 Hz or 60 Hz input, may be used as a frequency reference for electronics devices to generate internal clocks. A 50 Hz AC input is common in Europe and Asia, while the 60 Hz AC input is common in America. The incoming AC signal is stepped down using a transformer and supplied to a real time clock (RTC) chip, such as that available from STMicroelectronics under part number M41T50. The stepped-down AC signal can also be rectified to provide a DC signal that is voltage-regulated and used to power the RTC chip. The RTC chip counts seconds, minutes, hours, days, etc. based on the received AC input to generate a real-time clock. In the case of the M41T50 chip, a configuration bit in the chip's register is used to select between a 50 Hz or 60 Hz input.